1. Field of the Invention
The present invention relates to ceramic coatings and, more particularly, to putty-like ceramic coating compositions, and a method of applying such compositions to ceramic or metallic substrates.
2. Description of the Prior Art
Protective ceramic coatings are useful in various applications. For example, such coatings may be used as thermal barriers applied to ceramic or metallic substrates subjected to high temperatures, as abrasion resistant coatings, and as conductive coatings applied to electrical resistance elements.
Traditionally, ceramic coatings are applied to ceramic or metallic substrates using one of two different processes. In the first process, a slurry of a ceramic and/or metallic filler and a cementitious or sinterable ceramic or metal binder is sprayed, in a suitable aqueous or organic solvent, to a desired thickness on the substrate. The substrate is then fired to remove the solvent and any organic binders, and to form or sinter the ceramic binder. In the second process, the so-called dry spray method, the ceramic and/or metallic ingredients are sprayed without solvent at a high velocity onto the substrate through a flame or plasma which causes the component materials to melt prior to impact with the substrate.
These conventional methods suffer from several shortcomings. For example, the traditional wet spray method is quite labor intensive and in situations where direct measurement is impossible, thickness control is difficult. In order to provide ceramic coatings of substantial thickness, several layers of coating, each approximately two to three mils thick, must be applied to the part to be coated. Prior to applying each subsequent coat, the part must be dried, i.e., the solvent must be evaporated, to prevent dripping and sagging of the coating and to prevent the solvent from becoming entrapped as subsequent layers are applied, which may result in bubbling when the coating is fired. After the coating is built up to a sufficient thickness, it is fired to a temperature sufficient to react the cementitious binder or to sinter the ceramic binder.
In the case of cementitious binders and especially in the case of sinterable binders, supplemental organic thickeners are needed to prevent settling, to control rheology, and to impart green strength to the coating so as to minimize shrinkage or cracking during drying. The use of these additives may necessitate slow curing of the coated part to remove organic decomposition products without disrupting the coating structure.
In addition, ceramic coatings applied using the traditional wet spray method are often applied from aqueous solutions, requiring increased drying times and imparting increased porosities to the resulting ceramic coatings after drying. Moreover, because the ceramic coatings are applied as aqueous solutions, it is extremely difficult to make repairs of damaged components. This is especially true when the part requiring repair is large and must be repaired in the field, e.g., an aircraft wing, or when the damaged portion of the coated part is difficult to access.
Using the conventional dry spray method, specialized equipment is required to apply the coatings. Although this equipment is usually available where the initial coating is applied, it may not be available at locations where coating repairs must be made. Also, the extreme heat and velocity used in this method can damage certain fillers incorporated in the ceramic formulations by oxidizing or distorting the fillers on the substrate surface as the composition is applied. Such damage can, in certain instances, impair the performance of the coatings.
Thus, various problems and disadvantages exist with prior art methods for applying ceramic coatings. These problems and disadvantages are overcome by the present invention.